Internal combustion engine electronic ignition control system

ABSTRACT

An electronic ignition control system for internal combustion engine, notably for motor vehicles, which comprises a rotary member revolving at the engine velocity and provided with at least two reference marks of which the positions correspond to the maximum ignition advance angle and to the minimum ignition advance angle, respectively, said reference marks defining at least one area on said rotary member, a sensor disposed in close vicinity of said rotary member so as to detect the moments of passage of said reference marks and at least one up-and-down counter for counting pulses, said rotary member further comprising a third reference mark separate from the first two reference marks aforesaid so as to define two successive areas (A = 12, 13; B = 13, 14) scanned in succession by said sensor, while a first up-and-down counter positively counts the pulses from a first clock system during the passage of said first area (A) and negatively counts, during the passage of the second area (B), the pulses from a second clock system adapted to emit pulses at a frequency programmable according to the desired ignition advance law, the resetting of said up-and-down counter being utilized for producing the ignition spark.

The present invention relates generally to electronic means forcontrolling the ignition of internal combustion engines, notably ofmotor vehicles.

The primary requirement in the control of an internal combustion engineis the possibility of generating ignition pulses shifted in relation tothe top dead center of the piston by a magnitude to be determined andwhich must be independent of the rotational cycle of the engine itself.

Devices of this character are already known, such as devices operatingaccording to the principle of inertia or centrifugal weights, vacuumchamber and switching contacts. The chief inconvenience of devices ofthis known type is the necessity of frequently adjusting them in orderto obtain satisfactory engine performances.

Another known device referred to as a fully electronic one is based onthe electronic calculation of the ignition advance angle and thedetection of series of pulses along the outer periphery of the engineflywheel in order to determine their positions. These devices involvethe use of a complex computer and the relatively great number ofreference marks to be provided on the engine flywheel increases the costof the flywheel and also of the detecting means associated therewith.Other electronic control devices have already been proposed, but theysuffer from the inconveniences of one or both of the above mentioneddevices (misadjustment or complexity).

It is the essential object of the device according to this invention toprovide the same function, that is, calculating the value of theignition advance angle and to control the occurrence of the ignitionspark at a predetermined time, without the above mentionedinconveniences, therefore with a maximum reliability and a high degreeof reproductibility of the setting, while using simple and thereforeeconomical means.

The ignition electronic control device according to this invention,comprising a rotary member driven at engine speed and carrying at leasttwo reference marks on which the particular positions correspond to themaximum ignition advance angle and to the minimum ignition advance anglein order to determine at least one area on said rotary member, a sensorlocated in the vicinity of said rotary member so as to detect themoments when said reference mark move past, and at least one up-and-downpulse counter, is characterised in that said rotary member comprises athird reference mark 14 separate from the first pair of reference marks12, 13, so as to define two successive areas (A = 12, 13; B = 13, 14)scanned in succession by said sensor 4 and that a first up-and-downcounter 22 positively counts the pulses of a first clock system 19, 20during the passage of the first area A and negatively counts during thepassage of the second area B, the pulses of another clock system 19, 27emitting pulses at a frequency adapted to be programmed according to thedesired ignition advance law, the resetting of said up-and-down counterbeing utilized for producing the ignition spark.

According to a preferred form of embodiment, for a four-cylinder,four-stroke engine, the device according to this invention comprisesmeans for detecting six references marks on the engine flywheel whichare divided into two series of three, the two series being identical dueto the symmetrical engine operation. These three reference marksdetermine two areas on the engine flywheel, namely a first area formeasuring the instantaneous velocity and a second area constituting thespark release area.

An up-and-down counter is charged by a clock during the passage of thefirst area and discharged by another clock during the passage of thesecond area. The ratio of the two clock frequencies, as will beexplained presently, may be selected as a function of the ignitionadvance angle and in such a manner that the passage through zero of thenegative counter corresponds to the ignition time. The last referencemarks permits the release of the ignition spark in case of failure ofthe system in order to ensure an emergency or breakdown operation and toreset the assembly for another determination.

From the foregoing it is clear that this first system affords asatisfactory reliability and a proper reproductibility since thereference marks provided for controlling the spark are carried by theengine flywheel. On the other hand, the relatively small number ofreference marks on the flywheel corresponds to a reasonable cost of thedevice.

Another section of the device according to this invention is designedfor calculating by itself the ignition advance angle. To obtain optimumperformances, this calculation must be such as to generate advanceangles α consistent with a system of curves as illustrated in FIG. 1 ofthe attached drawings, i.e. of which the variations are linked to theengine velocity V and to the pressure P in the induction manifold. Thissystem of curve is a synthesis of the various ignition advance laws thatcan be encountered in the case of a standard engine. The law linking theignition advance to the vacuum prevailing in the induction manifold maybe constant, increasing, decreasing, or alternating, and its pattern maychange when the engine velocity varies. Practical experiments provedthat when this law has a breaking point, its abscissa Po is the same forall engine speeds. On the other hand, both the value of the advanceangle for P = Po and the various gradients are any functions ofvelocity. All these remarks explain the trend of the curves of FIG. 1.As a rule, the construction of this system of curves requires the use ofa complex calculation unit if the parameters are measured in aconventional way. The second portion of this invention relates to ameasuring and calculation unit capable of solving this problem in aparticularly simple manner.

From the system or network of curves of FIG. 1, it is apparent that thevalue of the advance angle may be divided into two partial values. Thefirst partial value is linked to the velocity referred to hereinafter asa (w) and the second partial value linked both to the velocity and tothe pressure may be expressed as the product of (P-Po) by a term b (w)denoting the gradient of the straight lines, this gradient varying as afunction of the velocity and of the (positive or negative) sign of(P-Po). The correction for temperature is limited as a rule to theincrement of the advance angle value below predetermined temperatures.The device according to this invention is further characterised in thatit comprises a pressure sensing member adapted to generate both the signand the absolute value of P-Po), and an elementary calculation deviceperforming the complementary operations. This calculation devicecomprises a memory for determining the velocity functions and a discretebinary multiplier for giving the necessary product. Finally, anup-and-down counter calculates the final value of the advance angle.With this device it is possible to provide the desired function withoutresorting to complex calculating or computing units, therefore at a costmaterially lower than that of hitherto known devices.

This invention will be better understood as the following descriptionproceeds with reference to the attached drawings illustratingdiagrammatically by way of example, not of limitation, a typical form ofembodiment thereof. In the drawings:

FIG. 1 is a system of curves showing the value of the ignition advanceangle α as a function of the induction pressure P and the enginevelocity V;

FIG. 2 illustrates in block diagram form the device according to thisinvention;

FIG. 3 shows a detail of the sensor means for detecting the position ofthe engine flywheel;

FIG. 4 illustrates the signal received from this sensor;

FIG. 5 illustrates in block form the ignition release member;

FIG. 6 illustrates the computing unit also in block form;

FIG. 7 illustrates the time diagram of the calculation;

FIG. 8 illustrates the base clock of the computer, and

FIG. 9 illustrates a typical form of embodiment of the pressure sensorand of the treatment member controlled thereby.

Referring to FIG. 2 of the drawings, there is illustrated therein ageneral or block diagram of the device of this invention, which isassociated with an internal combustion engine 1 comprising a combustionchamber 2 and a flywheel 3. Three data are measured: the passages ofreference marks on the flywheel 3 are detected by a position sensor 4;the pressure in the induction manifold is measured by another sensor 5,and the engine temperature is measured by a temperature sensor 7. Thesethree data are fed to a computer 8 adapted to deliver at its output 9 apulse for controlling a high-voltage system 10 for operating a sparkingplug 11.

FIG. 3 illustrates the details of the system for detecting the positionof the engine flywheel 3. The sensor 4 operating according to a methodbased on the magnetic-field detecting effect delivers a pulse each timeone of the magnets fitted in notches 12, 13 and 14 moves past saidsentor 4. The magnets are so disposed that the polarity of the magnet innotch 12 is opposite that obtaining in notches 13 and 14. The pulsesreceived at the output S of sensor 4 are illustrated in the diagram ofFIG. 4; thus, these pulses are positive for example for notch 12 andnegative for notches 13 and 14. The position of these notches on theflywheel is such that the area A (FIG. 3) corresponds to a predeterminednumber X of degrees on the flywheel. Point E designates the maximumpossible value of the advance angle, and point C its minimum value. Itmay be seen that another set of three identical notches is disposeddiametrally opposite this set on the flywheel for providing thenecessary two ignitions per revolution for a four-cylinder, four-strokeengine. This sensor, providing the three pulses denoted SD, SE and SC(FIG. 4) in connection with the generating notches illustrated in FIG.3, in which the pulse SD differs from the other pulses, may beconstructed in a different manner without departing from the basicprinciple of the invention. Similarly, it is also possible to use asensor capable of detecting the three reference marks 12, 13 and 14without detecting the difference between these marks, this detectionbeing achieved by a second sensor.

FIG. 5 illustrates the device contemplated for releasing the ignitionpulse. The signal S is fed to the input of a decoder 15 for separatingthe three signals SD, SE and SC and delivering them from three differentoutputs. Signals SD and SE are fed to a flip-flop univibrator 16 adaptedto open a gate 18 with its output 17 in the gap separating the twosignals. Thus, the signal delivered by a clock 19 and subsequentlydivided in a counter 20 operating as a divided by N can be fed to thecounting input 21 of an up-and-down counter 22.

As the signal SE appears, the signal at 21 disappears and a flip-flopunivibrator 23 opens by means of its output 24 a gate 25 allowing theclock signal to be fed to the input of the aforesaid up-and-down counter22 after having been divided in a preselection counter 27 operating as aprogrammable divider. This counter 27 has programmation inputs 28receiving in binary form a number N1 proportional to the advance angleand delivered by an ignition advance angle computer illustrated in FIG.6; in other words, the division of factor of counter 27 is thecomplement between N1 and its counting capacity. When the up-and-downcounter 22 clears its zero position, its output 29 emits a pulserestoring the flip-flop univibrator 23, thus closing the gate 25 and,via an OR gate 30 and its output 31, operates the high-voltage device 10(FIG. 2) producing the ignition spark. In other words, the mode ofoperation of the device may be so selected that the variable ratio ofthe division factors of counters 27 and 20 determines the value of theangle fraction of area A which follows the point E when the ignition isproduced, and the higher N1, the lower this value.

A safety device consists of a trigger 32 delivering at its output 33 apulse transmitted to output 31 via said OR gate 30. This detector 32generates a pulse when the signal SC appears before the output signalfrom down-counter 22, i.e. the spark signal. In fact, in this specificcase, the spark is issued from area B (FIG. 3) in which it shouldnormally be generated. Therefore, the apparatus made an erroneousdetermination and the spark must occur at SC for operating the engine.This device actually consists of an automatic emergency means for thecomputing system. Finally, the signal SC resets all the counters of thedevice to enable same to receive the next signal SD, but this has notbeen illustrated in the drawings for the sake of clarity.

The computing unit illustrated in FIG. 6, which generates a value of N1proportional to the advance angle, comprises a memory unit 34 consistingeither of a so-called "dead" memory or of a programmable logic unit, orany decoder capable of performing the desired function. This device hastwo inputs 35 and 36 to which digit signals are fed, and outputs 37, 38and 39 where likewise digit signals appear, thus causing the value ofthe output signals to be linked to that of the input signals by aprogram pre-recorded in the memory unit 34.

Connected to the inputs 35 are the outputs of a counter 40 receiving inturn at its counting input 41 the signal S. This counter 40 operatesduring a fixed time period determined by a signal H1 fed to input 42.The signal H1 is delivered by an internal clock of the system of which atypical example will be described presently with reference to FIG. 8.The counter outputs will thus transmit a number proportional to theengine velocity, and the numbers delivered by the memory unit 34 areproportional to the engine speed. It may be seen that an alternatesolution consists in feeding the signal S to the initializing input 42and then to count the pulses from a fixed clock during a time equal tothe period of rotation, whereby the output signal will be proportionalto the inverse of the engine velocity. The choice between these twosolutions is dictated by a compromise between accuracy and rapidity.

The signal P from the pressure sensor 5 (FIG. 2) is fed to an interfaceelement 43 to be described presently in detail. This interface comprisestwo outputs 44 and 45. The output 45 delivers a signal F having afrequency proportional to the absolute value of (P-Po) wherein Pdesignates the pressure in the induction manifold 6 (FIG. 2) and Po afixed value of said pressure, which appears in FIG. 1. The output 44 isa signal denoting the sign of (P-Po) which is fed to the input 36 ofmemory unit 34.

The outputs of this memory unit 34 are connected as follows: the output37 transmitting a binary number denoted b (w) which is a function ofvelocity or the inverse thereof, according to the specific caseinvolved, is connected to the input of a discrete multiplier 46delivering at its output a signal F1 and receiving at another input thesignal F having a frequency proportion to (P-Po), as explainedhereinabove. The signals are such that the frequency of signal F1corresponds to the relationship:

    F1 = F × b (w)

wherein all the terms are now known. The signal F1 is fed through a gate48 during a predetermined time to one input of an up-and-down counter47. The function of gate 48 is to select the input of the down-counterto which the signal F1 is to be fed according to the value (0 or 1) ofthe signal available at the output 38 of the memory unit 34 andrepresentative of the sign of b (w), and also to determine the timeduring which said signal F1 will be fed by virtue of its input 49receiving the signal H2 from the internal clock of the systemillustrated in FIG. 8. The aforesaid gate 48 will thus comprise an ANDcircuit 72 receiving at its inputs the signals F1 and H2, the output ofthis circuit being connected to one of the inputs of each one of a pairof AND circuits 73 and 74 having their outputs arranged to form a gateat 50 and 51. At the second input of said AND circuit 73 the signal 0 or1 of said output 38 is fed, the same signal being inverted in a NOcircuit 75 before being also fed to the second input of said AND circuit74. Therefore, according to the value of the signal obtaining at saidoutput 38 the signal F1 is switched towards the output 50 or output 51of said gate, provided that the signal H2 be present for permitting thispassage. If signal F1 is present at output 50 of gate 48, it is feddirectly to the down-counting input of said up-and-down counter 47. Inthe other alternative, it is present at output 51 of said gate and fedto the up-counting input 52 of up-and-down counter 47 via an OR gate 53.

The up-and-down counter 47, in addition to the above-mentioned up-anddown-counting inputs, receives at its pre-section or predeterminationinputs 54 the aforesaid number a (w) from ouputs 39 of the memory unit.This number, like b (w), is proportional to the engine velocity or tothe inverse thereof, according to cases. When the signal H3 from theclock of FIG. 8 is fed to the loading input 55 of said up-and-downcounter 47, the number a (w) is entered into this up-and-down counter.Any pulse fed to the up-and-down counting inputs is thus added to orsubstracted from this number, to display at the outputs 56 the value ofN1, which is the result of the calculation.

The signal T from the temperature sensor 7 (FIG. 2) is shaped in ashaping network 57 and causes pulses to be emitted at the output 58 of agenerator 58 of a generator 59 at predetermined temperature values.These pulses are fed through the OR gate 53 to the input of saidup-and-down counter 47.

Preliminary to the description of the mode of operation of the device ofthis invention, reference must be made again to the calculationrelationship given in the preamble of the description with reference toFIG. 1. In fact, the general relationship required for reproducting theset of curves of FIG. 1 has the form:

    α = a (w) + b (w).sup.· (P-Po)

in which

α is the value of the advance angle,

a (w) is a constant of which the value depends on w (engine velocity),

b (w) is the gradient of the straight lines of the diagram, its valuebeing subordinate to both the engine speed (w) and the sign of (P-Po),

P is the value of the pressure in the engine induction manifold, and

Po is the specific value of this pressure which appears in the diagramof FIG. 1.

The memory unit 34 contains the above-defined values a (w) and b (w).Its input 35 receives the value of the velocity or its inverse value,these two solutions differring from each other for the values of a (w)and b (w) only by the internal coding of the memory unit 34. The otherinput 36 receives the sign of (P-Po) as explained in the foregoing. Itis thus clear that the values available at 37, 38 and 39 may berespectively the value of b (w), the sign of b (w) which is positiveunder all circumstances. For making a calculation, the up-and-downcounter 47 is charged with a (w), then a number of pulses proportionalon the one hand to the product (P-Po)· b (w) generated by the discretebinary multiplier 46 and on the other hand to the temperature viagenerator 59, is added or deducted. The number N1 obtained at the end ofthe cycle at the output of the up-and-down counter 37 is actually thedesired result. The sync signals mentioned in the specification andreproduced in the diagram of FIG. 7 are generated by the basic clock ofthe system, which is illustrated in FIG. 8. In this Figure, as well asin FIG. 7, it will be seen after the spark has been struck (which, asalready explained, took place between SE and SC), the signal SC causesthrough its ascending front the signal H1 to appear, this signal H1having a duration t1 and causing in turn the release of two signals H2and H3, signal H2 having a duration t2 and signal H3 an extremely shortduration compared with t2.

The time t1 is the time necessary for counting the velocity information,and the time t2 is the time necessary for counting the product b (w).sup.· (P-Po). A typical clock construction is illustrated in FIG. 8.The signal SC is fed to the input 60 of a monostable multivibrator 61generating at its output a signal H1. This signal H1 is fed in turn toanother pair of monostable multivibrators 62 and 63 at their inputsresponsive to the descending or trailing end of the pulses. Thesevarious monostable multivibrators generate the requisite signals H2 andH3. If the counter 40 has at its output the inverse of the velocityvalue, the time t1 is eliminated and consequently the monostablemultivibrator 61 is also eliminated. The signal SC operates or triggersthe monostable multivibrators 62 and 63. Thus, signal H1 is replaced bya signal H'1 equal to the time elapsing between SE and SC (see FIG. 7)produced in a known manner by means of a flip-flop, this signal is infact proportional to the inverse of the rotational velocity of theengine, in order to yield the desired function.

FIG. 9 illustrates one of the possible forms of embodiment of thepressure sensor and of the interface required for obtaining thecharacteristic features of the device of this invention.

The pressure sensor 5 illustrated comprises a case of rigid metaldivided into two chambers by a diaphragm 64. One chamber communicateswith the indication manifold or pipe 6 of the engine, and the otherchamber is vacuumized. Therefore, the diaphragm position is subordinateto the absolute pressure P prevailing in the induction manifold. Thisdiaphragm has its central portion rigidly connected to the movable coreor plate 65 of a capacitor having its fixed sheath or plate 66 sodisposed that the maximum capacity value C of the capacitor be obtainedwhen P = Po. On either side of this value, the capacity decreases. Thecapacitor thus constructed controls the operation of an oscillator 67disposed in the above-defined interface 43. With this arrangement, theoscillator 67 is adapted to deliver at its output 45 a signal F having afrequency proportional to the absolute value of (P-Po). On the otherhand, an optical-electronic device consisting of an emitter 68 and areceiver 69 is adapted to determine the position of the movable element65 of the capacitor. When this movable element 65 has moved beyond thepoint of maximum capacity (that is, when this element 65 is retractedcompletely within the sheath 66), the path of the light beam 70 is freeof any obstacle and the amplifier 71 receives a signal which is thusamplified and delivered at its output 44. This signal is therefore theequivalent of the sign of said quantity (P-Po).

Now, it will readily occur to those conversant with the art that thisspecific form of embodiment is given by way of example only, for manyother solutions may be brought to this problem for obtaining the sameresult, from the dual point of view of the method of measuring thenegative pressure and of the method of measuring the collectedinformation in the form of electric signals.

Besides, although a specific form of embodiment of this invention hasbeen described hereinabove and illustrated in the accompanying drawings,it will readily occur to those skilled in the art that variousmodifications and changes may be brought thereto without departing fromthe scope of the invention as set forth in the appended claims.

What is claimed as new is:
 1. an electronic ignition control system foran internal combustion engine comprising a rotary member means forrevolving at the engine velocity, two successive reference markspositioned on said rotary member means, a sensor means disposed in closevicinity of said rotary member means for producing successively a firstand second signal at the moment of passage of said two reference marks,a decoder means, means for connecting said sensor means to said decodermeans, said decoder means provided for separating respectively on afirst and second output, said first and second signal, an up-and-downcounter, a first clock means for producing pulses which are gated forpositive counting to said up-and-down counter during the time separatingsaid first and second signals, a second clock means for producing pulseswhich are gated for negative counting to said up-and-down counter inresponse to said second signal, said first and second clock means havinga common clock and said second clock means including a preselectioncounter operable as a programmable divider comprising programmationinputs, an ignition advance angle computing means for generating anumber representative of a desired ignition advance law characteristics,means for applying said number to said programmation inputs, means forre-setting said up-and-down counter, and generating means responsive tothe resetting of said up-and-down counter for interrupting said negativecounting and for generating an ignition signal, wherein said rotarymember means includes a third successive reference mark, said sensormeans including means for producing a third signal at the moment ofpassage of said third reference mark, said decoder means including meansfor separating on a third output said third signal, said generatingmeans for generating an ignition signal including an OR gate having afirst input responsive to said resetting of said up-and-down counter anda second input which is operatively connected to the third output ofsaid decoder so that said OR gate generates an ignition signal in caseof failure of the signal applied to its first input.
 2. An electronicignition control system according to claim 1 wherein said referencemarks include permanent magnets, the polarity of one reference markopposite to that of the other two reference marks for detecting theorder in which said reference marks move past said sensor means.
 3. Anelectronic ignition control system for an internal combustion enginecomprising a rotary member means for revolving at the engine velocity,two successive referenc marks positioned on said rotary member means, asensor means disposed in close vicinity of said rotary member means forproducing successively a first and second signal at the moment ofpassage of said two reference marks, a decoder means, means forconnecting said sensor means to said decoder means, said decoder meansprovided for separating respectively on a first and second output, saidfirst and second signal, and up-and-down counter, a first clock meansfor producing pulses which are gated for positive counting to saidup-and-down counter during the time separating said first and secondsignals, a second clock means for producing pulses which are gated fornegative counting to said up-and-down counter in response to said secondsignal, said first and second clock means having a common clock and saidsecond clock means including a preselection counter operable as aprogrammable divider comprising programmation inputs, an ignitionadvance angle computing means for generating a number representative ofa desired ignition advance law characteristic, means for applying saidnumber to said programmation inputs, means for re-setting saidup-and-down counter, and generating means responsive to the resetting ofsaid up-and-down counter for interrupting said negative counting and forgenerating an ignition signal, wherein each of said first and secondclock means comprises and AND gate having an input connected to saidcommon clock, the AND gate of said first clock means having a secondinput connected to the output of a first flip-flop, the two inputs ofsaid flip-flop respectively connected to the first and second outputs ofsaid decoder means, the AND gate of said second clock means having asecond input connected to the output of a second flip-flop, the twoinputs of said second flip-flop respectively connected to the secondoutput of said decoder means and to said up-and-down counter so as to bereponsive to the resetting of said up-and-down counter.
 4. An electronicignition control system for an internal combustion engine comprising arotary member means for revolving at the engine velocity, two successivereference marks positioned on said rotary member means, a sensor meansdisposed in close vicinity of said rotary member means for producingsuccessively a first and second signal at the moment of passage of saidtwo reference marks, a decoder means, means for connecting said sensormeans to said decoder means, said decoder means provided for separatingrespectively on a first and second output, said first and second signal,an up-and-down counter, a first clock means for producing pulses whichare gated for positive counting to said up-and-down counter during thetime separating said first and second signals, a second clock means forproducing pulses which are gated for negative counting to saidup-and-down counter in response to said second signal, said first andsecond clock means having a common clock and said second clock meansincluding a preselection counter operable as a programmable dividercomprising programmation inputs, an ignition advance angle computingmeans for generating a number representative of a desired ignitionadvance law characteristic, means for applying said number to saidprogrammation inputs, means for re-setting said up-and-down counter, andgenerating means responsive to the resetting of said up-and-down counterfor interrupting said negative counting and for generating an ignitionsignal, wherein said ignition advance angle computing means comprises amemory means having inputs and a plurality of outputs for producingdigital output signals, said memory means including a pre-recordedprogram for processing input signals applied to said inputs, whereinsaid digital output signals are linked to said input signals, a counter,an activation signal generator, means for connecting said activationsignal generator to said counter, said sensor means connected to saidcounter, wherein the counter output signals are related to the enginespeed, and means for applying said counter output signals to the inputsof said memory, a vacuum pressure sensor means for sensing the pressurein the induction manifold of the engine, an interface means connected tosaid vacuum pressure sensor means for producing separately a firstsignal having a frequency proportional to the absolute value of (P-Po),wherein P is the pressure measured in the induction manifold and Po apredetermined intermediate value of the pressure range in the inductionmanifold, and a second signal representing the sign of (P-Po), saidsecond signal applied to said memory, a discrete multiplier having oneinput connected to said interface means to receive said first signalemitted by said interface means and additional inputs connected to agroup of outputs of said memory, said outputs of said memory deliveringdigital signals depending on the engine speed, a gate connected to theoutput of said multiplier and connected to a further output of saidmemory, said further output of said memory delivering digital signalsdepending on the engine speed, a gate connected to the output of saidmultiplier and connected to a further output of said memory, saidfurther output of said memory representative of the sign of (P-Po), saidgate having two gate outputs and means for activating said gate outputsaccording to the sign of (P-Po), an up-and-down preselection countermeans comprising predetermination inputs and means for connecting saidpredetermination inputs to a corresponding group of outputs of saidmemory, said corresponding group of outputs of said memory deliveringdigital signals in relation to the engine speed, up-counting anddown-counting inputs, and means for connecting said up-counting anddown-counting inputs to respective outputs of said gate, and outputs ofsaid preselection counter means defining said number representative ofthe desired ignition advance law and connected to the programmationinputs of said preselection counter operable as a programmable divider,and further activation signal generators for periodically activatingsaid gate and the loading of the up-and-down preselection counter means,to produce said number representative of the desired ignition advancelaw.
 5. An electronic ignition control system according to claim 4,further comprising an engine temperature sensor means, a pulse generatorconnected to said temperature sensor means for producing output pulsesin relation to the engine temperature, and an OR gate interposed betweenthe up-counting input of said up-and-down preselection counter means andthe corresponding output of said gate, said OR gate having an inputconnected to the output of said pulse generator.
 6. An electric ignitioncontrol system according to claim 5, wherein said vacuum pressure sensormeans comprises a diaphragm responsive to said pressure, a variablecapacitor having a movable core, said movable core connected to saiddiaphragm, wherein the maximum capacity value for said variablecapacitor occurs for P = Po, and a decreasing capacity on either side ofthis value, said variable capacitor comprising means for cooperatingwith an oscillator to produce said first signal having a frequencyproportional to (P - Po), and said movable core cooperating with anoptical-electronic means for sensing the position of said movable corefor determining the sign of (P - Po).